1. Field of the Invention
The present invention relates to a bone fixation device and, more particularly, to a bone fixation device for vertebrae which includes a sliding cover and bone screw with a special thread.
2. Description of the Related Art
In the art of orthopedic surgery, and particularly in spinal surgery, it has long been known to affix an elongated member, such as a plate or rod, to bones in order to hold them and support them in a given position. For example, in a procedure to fuse damaged, diseased, malformed or otherwise abnormal vertebrae, the vertebrae are positioned in a corrected position by a surgeon. An elongated plate is placed adjacent to the vertebral bone, and bone anchors, such as specially-configured screws or bolts, are employed to secure the plate to the bones. With such anchors placement is accomplished by drilling one or more holes in the bone(s), and threading the anchors into the holes. An anchor can be connected to the bone, as by threading into a vertebral hole, through a plate, or alternatively the plate can be placed in position over or around the anchor after the anchor is connected to the bone. The anchor and plate are then secured to each other to minimize or prevent relative movement. In this way, bones may be spinal held and/or supported in proper alignment for healing. However, since the vertebrae can move at multiple angles, forces at different directions or angles generated by the relative movement between the bones can push and turn the screws in the vertebral holes, and causing them to pull out.
Furthermore, the typical bone screw has the same thread rotation angle along its entire body; when force generated by the relative movement between the bones pushes the screw, or the tip end of the thread reaches the edge of the through hole of the bone plate, the screw can be easily pushed or turned.
Therefore, it is desirable to provide a bone fixation device to mitigate and/or obviate the aforementioned problems.